1. Field of the invention
The present invention relates to a composition for skin care, which is identical in lipid composition and structural properties to intercellular lipid lamella of stratum corneum.
2. Description of the Prior Art
In terms of structure, the skin can be divided into dermis, epidermis, and stratum corneum in accordance with the differentiation of keratinocyte cells from a basal layer. Particularly, the stratum corneum, which is the outermost layer of the skin, is primarily responsible for protecting the skin from external physical and chemical damage as well as functioning as a barrier to prevent the loss of internal water.
Deeply understanding the structure of the skin and the physical and chemical properties of epidermal lipid constituents is indispensable for and helpful in developing skin applications for maintaining the skin in healthy condition.
Since dry skin is a cause of various dermatological troubles, to maintain skin""s moisture balance is fundamental to skin care. Also, the skin is exposed to numerous microorganisms and the stratum corneum is known to have components which act to maintain normal bacterial flora on the skin.
Phospholipids, one of the important skin lipids constituting the lipid lamella bilayer, are quantitatively present in the basal layer. However, phospholipids are present in lower proportions in the lipids of more outer epidermal layers. On the other hand, ceramides become more plentiful in more outer layers, amounting to as much as 50% of the intercellular lipids in the outer protective layers of skin, stratum corneum.
It is generally accepted that ceramides play a particularly important role in preventing the loss of the water through the stratum corneum and restoring any damaged skin lipid barrier of the stratum corneum. It is also known that phytosphingosine, a ceramide precursor, is formed as a result of the degradation of the ceramides of the stratum corneum and functions as a primary microorganism barrier to inhibit the growth of microorganisms on the outermost layer of the skin and to reduce skin troubles. A reduction of the ceramide content in the stratum corneum brings about a decrease of phytosphingosine content, leading to skin troubles such as acne and atopic dermatitis and giving off offensive odors.
As various skin diseases, including atopic dermatitis, are reported to be attributed to abnormal changes in the stratum corneum lipid composition, there is a new tendency to develop products from combinations of ceramides, cholesterol and fatty acids in the dermatology and cosmetic fields.
For these reasons, extensive research is concentrated on the development of skin care products taking advantage of stratum corneum lipids and phytosphingosine. In order to exhibit sufficient physiological activity, these substances must penetrate through the stratum corneum to the deeper layers of skin, such as epidermis and dermis, composed of a larger proportion of phospholipids. To this end, lipid lamellar type products must be developed to have structures similar to those of the lipid lamellar of the stratum corneum.
However, since ceramides and phytosphingosines are hydrophobic materials, they are low in solubility and highly apt to form crystals. Therefore, ceramides and phytosphingosines are poor in stability in cosmetic formulations and limited in their applications. Recently, ceramide mixtures formed with the help of ethanol/propylene glycol have been reported to recover damaged skin to a normal condition, but it is also reported that their use for a long period of time makes the skin dry, and propylene glycol can so much as cause contact allergy. On account of, the use of solvents and additives unsuitable for use on the skin, the conventional skin care compositions cause side effects and show poor trans-epidermal penetration. In order to solve the above-mentioned problems, there have been developed pseudo-ceramide comprising products which are of high solubility and facile utility. However, pseudo-ceramides, which are not in natural form, may cause problems in that they are easily accumulated in the skin because they are not biodegradable. More importantly, pseudo-ceramides lack biological activities that the natural ceramides or phytosphingosine retain.
Therefore, a specific formulation is required in order to sufficiently exert the effects of ceramides and phytosphingosines on the skin. As a solution to these problems, the present inventors have developed a composition for skin care, which is superior in skin compatibility, that is, which is similar in composition and structural properties to that of human skin.
It is an object of the present invention to provide a skin care composition which is similar in composition and structural properties to that of human skin, thereby to sufficiently exert the efficacies of ceramides and phytosphingosine and a preparing method thereof.
The composition of the present invention comprises ceramides, cholesterol and fatty acids, which are major intercellular components of stratum corneum, phospholipids which are major components of epidermal cell membranes, and phytosphingosine and its derivatives, which serve as physiologically active materials.
The ceramides useful in the present invention comprise derivatives of sphingosine, sphinganine and phytosphingosine, and mixtures thereof.
Available in the present invention is phytosphingosine itself. Polar derivatives are, however, preferred in order to enhance the emulsification and solubility of phytosphingosine.
For example, organic acids are used to produce organic salts of phytosphingosine which are neutral in pH. Available organic acids are exemplified by hydrogen chloride, lactic acid, xcex1-, or xcex2-hydroxy acid, and salicylic acid.
As the fatty acids in the skin care composition of the present invention, saturated or unsaturated fatty acids containing 10-25 carbon atoms or combinations thereof are used.
Functioning as a skin penetration enhancer and an emulsifier, phospholipids used in the present invention are preferably hydrogenated or hydroxylated to contribute to the stability of the composition.